1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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3 | <!ENTITY % general-entities SYSTEM "../general.ent">
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4 | %general-entities;
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5 | ]>
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6 | <sect1 id="ch-scripts-usage">
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7 | <title>How Do These Bootscripts Work?</title>
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8 | <?dbhtml filename="usage.html"?>
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9 |
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10 | <indexterm zone="ch-scripts-usage">
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11 | <primary sortas="a-Bootscripts">Bootscripts</primary>
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12 | <secondary>usage</secondary></indexterm>
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13 |
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14 | <para>Linux uses a special booting facility named SysVinit that is
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15 | based on a concept of <emphasis>run-levels</emphasis>. It can be quite
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16 | different from one system to another, so it cannot be assumed that
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17 | because things worked in <insert distro name>, they should work
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18 | the same in LFS too. LFS has its own way of doing things, but it
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19 | respects generally accepted standards.</para>
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20 |
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21 | <para>SysVinit (which will be referred to as <quote>init</quote> from
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22 | now on) works using a run-levels scheme. There are seven (from 0 to 6)
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23 | run-levels (actually, there are more run-levels, but they are for
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24 | special cases and are generally not used. The init man page describes
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25 | those details), and each one of those corresponds to the actions the
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26 | computer is supposed to perform when it starts up. The default
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27 | run-level is 3. Here are the descriptions of the different run-levels
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28 | as they are implemented:</para>
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29 |
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30 | <literallayout>0: halt the computer
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31 | 1: single-user mode
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32 | 2: multi-user mode without networking
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33 | 3: multi-user mode with networking
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34 | 4: reserved for customization, otherwise does the same as 3
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35 | 5: same as 4, it is usually used for GUI login (like X's <command>xdm</command> or KDE's <command>kdm</command>)
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36 | 6: reboot the computer</literallayout>
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37 |
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38 | <para>The command used to change run-levels is <command>init
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39 | <replaceable>[runlevel]</replaceable></command>, where
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40 | <replaceable>[runlevel]</replaceable> is the target run-level. For
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41 | example, to reboot the computer, a user would issue the <command>init
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42 | 6</command> command. The <command>reboot</command> command is an
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43 | alias for it, as is the <command>halt</command> command an alias for
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44 | <command>init 0</command>.</para>
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45 |
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46 | <para>There are a number of directories under <filename
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47 | class="directory">/etc/rc.d</filename> that look like <filename
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48 | class="directory">rc?.d</filename> (where ? is the number of the
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49 | run-level) and <filename class="directory">rcsysinit.d</filename>, all
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50 | containing a number of symbolic links. Some begin with a
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51 | <emphasis>K</emphasis>, the others begin with an
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52 | <emphasis>S</emphasis>, and all of them have two numbers following the
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53 | initial letter. The K means to stop (kill) a service and the S means
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54 | to start a service. The numbers determine the order in which the
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55 | scripts are run, from 00 to 99—the lower the number the earlier it
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56 | gets executed. When init switches to another run-level, the
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57 | appropriate services get killed and others get started.</para>
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58 |
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59 | <para>The real scripts are in <filename
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60 | class="directory">/etc/rc.d/init.d</filename>. They do the actual
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61 | work, and the symlinks all point to them. Killing links and starting
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62 | links point to the same script in <filename
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63 | class="directory">/etc/rc.d/init.d</filename>. This is because the
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64 | scripts can be called with different parameters like
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65 | <parameter>start</parameter>, <parameter>stop</parameter>,
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66 | <parameter>restart</parameter>, <parameter>reload</parameter>, and
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67 | <parameter>status</parameter>. When a K link is encountered, the
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68 | appropriate script is run with the <parameter>stop</parameter>
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69 | argument. When an S link is encountered, the appropriate script is run
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70 | with the <parameter>start</parameter> argument.</para>
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71 |
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72 | <para>There is one exception to this explanation. Links that start
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73 | with an <emphasis>S</emphasis> in the <filename
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74 | class="directory">rc0.d</filename> and <filename
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75 | class="directory">rc6.d</filename> directories will not cause anything
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76 | to be started. They will be called with the parameter
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77 | <parameter>stop</parameter> to stop something. The logic behind this
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78 | is that when a user is going to reboot or halt the system, nothing
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79 | needs to be started. The system only needs to be stopped.</para>
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80 |
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81 | <para>These are descriptions of what the arguments make the scripts
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82 | do:</para>
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83 |
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84 | <variablelist>
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85 | <varlistentry>
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86 | <term><parameter>start</parameter></term>
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87 | <listitem><para>The service is started.</para></listitem>
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88 | </varlistentry>
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89 |
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90 | <varlistentry>
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91 | <term><parameter>stop</parameter></term>
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92 | <listitem><para>The service is stopped.</para></listitem>
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93 | </varlistentry>
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94 |
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95 | <varlistentry>
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96 | <term><parameter>restart</parameter></term>
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97 | <listitem><para>The service is stopped and then started again.</para></listitem>
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98 | </varlistentry>
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99 |
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100 | <varlistentry>
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101 | <term><parameter>reload</parameter></term>
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102 | <listitem><para>The configuration of the service is updated.
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103 | This is used after the configuration file of a service was modified, when
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104 | the service does not need to be restarted.</para></listitem>
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105 | </varlistentry>
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106 |
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107 | <varlistentry>
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108 | <term><parameter>status</parameter></term>
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109 | <listitem><para>Tells if the service is running and with which PIDs.</para></listitem>
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110 | </varlistentry>
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111 | </variablelist>
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112 |
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113 | <para>Feel free to modify the way the boot process works (after all,
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114 | it is your own LFS system). The files given here are an example of how
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115 | it can be done.</para>
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116 |
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117 | </sect1>
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118 |
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